The manufacturing process of rotors of generators used in large scale modern wind turbines for instance, in particular direct drive generators for gearless types of wind turbines, comprises in accordance with prior art: rolling a plate of steel with a thickness of approximately 40 mm for instance to a circumferentially open circular cylinder having the desired outer diameter, welding the two adjacent sides of the cylinder so as to form a circumferentially closed cylinder followed by subsequent processing of the cylinder in a special lathe for multiple machining process steps until the desired dimensions of the rotor are achieved under consideration of given tolerances. The inner diameter tolerances may be +/−0.5 mm for an outer rotor type generator having a diameter of about 4 m for instance, which corresponds to permitted deviations of about 0.0125%. These process steps are time consuming and expensive and further take away some of the usability of the generator despite of the other advantages generators of the above described type usually have.